The present invention relates to methods and devices for sensing atmospheric humidity and is particularly concerned with the use of hygroscopic organic polymer membranes in such applications.
The use of organic polymers for humidity sensing is well known, and various devices incorporating organic polymer humidity sensing elements have been proposed in the prior art. Basically speaking, humidity sensing devices of the foregoing type operate by the detection of a physical property of the polymer which varies as a function of the water content of the polymer (i.e., which varies as water is adsorbed and desorbed with changing humidity conditions). Because the water content of the polymer is directly related to relative humidity at any given temperature, the value of the detected property at a particular temperature will provide a direct indication of relative humidity.
In perhaps the most common form of polymeric humidity sensor, an electrical property of the sensor is detected to determine humidity--most often resistance or conductance (the reciprocal of resistance), although capacitive type sensors are also known. Polymeric electronic humidity sensors are generally fabricated from a class of polymers known as polyelectrolytes or ion exchange resins. These materials are characterized by an insoluble polymer backbone with attached functional groups capable of exchanging one ion for another of similar type (e.g., cation for cation). The ion exchange capabilities of these materials are not actually utilized for humidity sensing. Rather, it is the characteristic change in properties of these materials as they adsorb and desorb water which is important. Exemplary functional groups of the cation exchange type include sulfonic, phenolic, phosphonic, and carboxylic acid groups. Conductivity among different polymers at a given temperature and water content will vary depending upon the particular functional group present, with the relatively strong acid groups such as sulfonic and phosphonic providing high conductivities and the relatively weak acid groups such as carboxylic and phenolic providing low conductivities.
Examples of polymeric electronic humidity sensors are given in U.S. Pat. No. 2,728,831 to Pope and U.S. Pat. No. 2,937,524 to Gregor. The sensors of these patents are hydrocarbon (e.g., polystyrene) based with sulfonic acid being a preferred functional group.
Other examples of organic polymers proposed for electronic humidity sensing include polyphenylacetylene (Hermans, "CO, CO.sub.2, CH.sub.4 and H.sub.2 O Sensing by Polymer Covered Interdigitated Electrode Structures," Sensors and Actuators, Vol. 5, pp. 181-186, 1984); cellulose acetate (Delapierre et al, "Polymer-Based Capacitive Humidity Sensor: Characteristics and Experimental Results," Sensors and Actuators, Vol. 4, pp. 97-104, 1983); and cellulose acetate butyrate (Misevich, "Capacitive Humidity Transducers," IEEE Trans. Ind. Electron. Conf. Instrum., pp. 6-12, 1969; Thoma et al, "A Capacitance Humidity-Sensing Transducer," IEEE Trans. Com. Hybrids Manuf. Tech., pp. 321-323, 1979). Notably, for reasons which will become apparent later, all of the foregoing polymers are hydrocarbons.
In spite of the variety of polymers available and the many improvements accomplished in recent years, state-of-the-art polymeric humidity sensors still suffer from a variety of significant disadvantages including hysteresis, non-linearity, instability, and lack of selectivity--all of which contribute to poor accuracy. State-of-the-art sensors are further limited in that they are generally characterized by short service life. Other problems associated with known organic polymer sensors include swelling and oxidation. Most significantly, these disadvantages become increasingly severe at elevated temperatures and high levels of relative humidity. Such temperature and humidity conditions frequently prevail in manufacturing environments, for example, in which humidity control may be required. An environment of this type might be the interior of a microwave oven or an industrial drier. Because of their disadvantages, polymeric humidity sensors have generally had little or no utility in such environments. Metal oxide (ceramic) humidity sensors exhibit similar disadvantages and have likewise been of only limited utility.
During the course of development of the present invention, many polymers were examined in an effort to produce a sensor which would provide high accuracy measurements with a long service life in high temperature-humidity conditions. In U.S. Pat. No. 4,083,765 to Lawson, an electrolytic hygrometer was disclosed incorporating a polymer sensing element having a long service life without exhibiting major performance deterioration. In particular, Lawson used a sensing element of perfluorocarbon polymer having pendant sulfonic acid groups. The element was of a tubular configuration with a pair of electrodes in contact with the inner and outer walls of the tube. A DC current through the element was measured as absorbed water was electrolyzed to hydrogen and oxygen to provide an indication of the water content of a test gas. Lawson did not consider use of the aforementioned polymer in an electronic thin-film humidity sensor. Developmental work in connection with the present invention, however, revealed that in such applications, this polymer is sensitive to variations in relative humidity only up to about 40 percent, with little change in response as humidity increases further. Thus, despite its improved service life, the polymer employed by Lawson, like others proposed heretofore, proved to be of little practical value for electronic humidity sensing applications in high temperature-humidity environments.